Inverters for converting DC power into AC power for energizing one or more loads typically include one or more inverter legs, each inverter leg in turn having a pair of controllable switches connected in series across the terminals of a DC power source. The inverter switches, which may be transistors or the like, may be operated by an inverter control circuit to produce a pulse width modulated, or PWM output at the inverter leg. This PWM output is converted into a sine wave by a filter and is utilized to energize the loads.
Prior inverter controls were capable of operating inverter switches to produce only one PWM output or pattern. However, it was found that changes in load power factor or variations in one or more operating parameters of the inverter resulted in the introduction of undesirable harmonics in the PWM output which require the use of a large output filter.
As a consequence of the above, inverter controls were developed which were capable of operating switches in an inverter to produce one of a plurality of PWM outputs. Such an inverter control is disclosed in Abbondanti U.S. Pat. No. 4,099,109. This patent discloses a memory in which is stored digital representations of a plurality of modulation patterns and means for addressing the memory in accordance with a signal to produce a desired one of the patterns. In a preferred form of this control, the memory stores digital signals representing a plurality of levels for each of a plurality of modulation patterns. These levels are sequentially developed at an output of the memory. Each level is compared to a digital ramp signal by a digital comparator. When the ramp signal reaches the level, a transition between zero and one levels in a modulation pattern is produced. The levels stored in the memory and the ramp signal therefore represent notch edges in the resulting PWM waveform.
The information stored in the memory represents the switching pattern required to reproduce one quadrant of a single phase output waveform. The switching patterns for the remaining quadrants and phases are derived by phase shifting and addressing code reversal.
One disadvantage of the Abbondanti control arises when it is used in a noisy environment. Should noise cause an alteration of the memory output, a large shift in the switching points for the PWM pattern can occur, thereby resulting in the introduction of significant distortion in the output.
Abbondanti suggests an alternative arrangement wherein a memory stores a series of zeroes and ones, with zeroes representing notches in the output and ones representing pulses in the output. The zeroes and ones are retrieved in real time to thereby produce the waveform sequentially by time increments. No circuitry, however, other than a memory is disclosed for accomplishing the above procedure.
Dougherty U.S. Pat. No. 4,348,734 discloses a digital controller for a power converter which includes a memory for storing digital representations of a plurality of switching patterns, each of which corresponds with a segment of a phase output waveform. A switching pattern is stored as a series of zeroes and ones in the memory, and the memory locations are sequentially accessed by means of an address generator having a ripple counter which accumulates pulses from a clock. The switching pattern is accessed in an appropriate fashion to synthesize the entire output waveform. The switching pattern segment lengths stored in the memory are disclosed as being in a range between 60.degree. and 90.degree. of the output, with the particular segment length being a function of the number of phases of the output power from the power converter.
Woehrle et al U.S. Pat. No. 4,290,108 discloses a control unit for a converter which operates in a fashion similar to the preferred embodiment disclosed by Abbondanti. Woehrle et al, in addition, discloses the use of a separate control means and memory for each of three phases to be produced by the converter so that independent control over each phase output can be accomplished. Also, this patent discloses that the transition points or notch edges for less than a whole period of the desired output waveform can be stored in the memory, a quarter period being preferred.
While each of the above patents discloses an inverter control which is capable of operating switches in a converter to produce one of a plurality of output waveforms, none of the art discloses such a control which can provide individual phase regulation and which is effective to maintain control over the output even in a noisy environment.